Door reinforcement beams serve as stiffeners for automobile doors by absorbing impact energy in the event of a lateral impact and to convert impact energy into mechanical work. In this manner, passengers in a car can be protected from injury. For such a door reinforcement to perform this safety function, the following values, for instance, must be satisfied:
Tensile strength Rm at least 1,100 N/mm.sup.2 PA1 Elastic limit Rt at least 800 N/mm.sup.2 PA1 Elongation at rupture A5 at least 8% or a work capacity W-150 of at least 1,900 Joules, depending on the dimensions of the section.
These minimum requirements can be lower or higher depending on the specification of the automobile manufacturer.
The work capacity of a door reinforcement beam can be tested by subjecting it to a bending of 150 mm in a 3-point bending test. The force applied is recorded with respect to the bending distance and the area below the curve is determined. This integral then gives the desired work capacity.
Since the door reinforcement should be suitable for installation in small cars having narrow doors, it is necessary that its dimensions be kept as small as possible. Furthermore, it is necessary that the total weight of the car be only minimally increased by the installation of the door reinforcement.
A door reinforcement beam of this type is known from Federal Republic of Germany Patent 36 06 024. The rectangular, preferably square, hollow section with bead portions which extend outward at both ends on the inner and outer flanges is an extended section of light metal. One particular embodiment as shown in FIG. 9 of the above referred patent is characterized by the fact that the central region of the impact beam is reinforced over a given length by a length of pipe arranged within it. This proposal, however, has the disadvantage that when light metal is used, the wall thicknesses of the section must be very large in order to obtain the desired work capacity. Furthermore, such a section is poorly adaptable to maneuvering around obstacles in an area of installation within the door. Furthermore, an expensive extrusion tool must be used with the section and once the tool has been selected for use with the section, it can no longer be used in the event of a structural change in the door or upon a change in the model.
An alternative solution can be noted from Federal Republic of Germany Patent 37 28 476 in which the values for the tensile strength and elastic limit of a door reinforcement tube are increased, as compared with the above-indicated prior art, by the use of a special steel alloy. This alternative however, has the disadvantage that the strength increasing effect is achieved only by alloying expensive elements such as nickel and molybdenum.